With popularization of front-engine front-wheel-drive vehicles for the purpose of achieving smaller and lighter weight vehicles and also increasing demands for larger riding spaces, automobiles are inevitably undergoing reduction in engine room space and thus requiring even smaller and lighter weight solutions for the electrical parts and engine accessories, such as alternators or idler pulleys, electromagnetic clutches for automotive air conditioners, and water pumps. In addition, it is increasingly desired for these individual parts to exhibit higher performance and greater output.
However, reduction in output with smaller size is unavoidable, and for example, increasing speed of alternators or electromagnetic clutches for automotive air conditioners compensates the reduction in output, which results in commensurate increase in idler pulley speed as well. Furthermore, since demands for improved silence have accelerated air-tightness of engine rooms and elevation of temperature in the engine room, such parts are required to have greater resistance against a high temperature of 180 to 200° C. Moreover, since automobiles are used in very cold land such as Alaska or Siberia, it is also necessary that the rolling bearings to be incorporated into the above individual parts should generate few trouble in rotation or less abnormal noise on start-up even under a cold condition of −30 to −40° C.
From such viewpoints, recently, grease compositions excellent in thermal stability, high-speed resistance, load resistance, and low-temperature properties have been developed and employed. For example, a diphenyl ether oil excellent in thermal stability is used as a base oil and a diurea compound excellent in heat resistance is used as a thickening agent, and on the other hand, it becomes the mainstream to use a base oil having a high viscosity in order to ensure a certain oil-film thickness in consideration of high load.
However, when the viscosity of a base oil increases, insufficient fluidity between a retainer of rolling bearings and rolling elements may occur on start-up at low temperature and a vibration noise may be sometimes generated at the retainer and outer ring. Moreover, since apparent viscosity of the grease composition also increases, grease is not fluidized between the rolling elements and the inner and outer rings, and thus lubrication conditions become insufficient, a vibration noise may be sometimes generated. In addition, since these vibration noises resonate with an attached site and are amplified into offensive abnormal noises, it becomes important to take measures to meet the situation especially at very cold land and the like.
In order to reduce the generation of such abnormal noises, there have been devised measures of sealing a grease composition wherein a mixed oil obtained by mixing an alkyldiphenyl ether oil and a poly(α-olefin) oil in a specific ratio is used as a base oil and a diurea compound is used as a thickening agent (see, Patent Document 1), of applying a lubricating oil such as a poly(α-olefin) oil onto a friction surface inside a bearing in an amount of 1.0 mg/cm2 or more (see, Patent Document 2), of applying an antirust lubricating oil containing an ester oil, a synthetic hydrocarbon oil, an ether oil, or the like as a main ingredient, which has a kinematic viscosity at 40° C. of 50 mm2/s or less (see, Patent Document 3), and the like measures.    Patent Document 1: Japanese Patent Unexamined Publication JP-A-5-140576    Patent Document 2: Japanese Patent Unexamined Publication JP-A-5-149343    Patent Document 3: Japanese Patent Unexamined Publication JP-A-11-22740